5. Choose the best description of a reversible reaction. a reaction that occurs in both directions at the same time
a reaction that has used up all the reactants and has stopped
a reaction that can only proceed to the right
a reaction that can only proceed to the left

A reaction that can only move towards the right.

Explanation:

For a reversible reaction when one of the products of the reaction is removed the reaction moves towards right hand side. More and more reactants will be converted into products. This will nullify the effect of removal of product.

the formula mass of a molecule or a compound is the sum of the atomic weights of the atoms in its empirical formula

(that's what i know about formula mass)